late 14c., from Medieval Latin triplare "to triple," from Latin triplus "threefold, triple" (see triple (adj.)). Related: Tripled; tripling.
early 15c., from Old French triple or directly from Latin triplus "threefold, triple," from tri- "three" (see tri-) + -plus "-fold" (see -plus). As a noun, early 15c., "a triple sum or quantity," from the adjective. The baseball sense of "a three-base hit" is attested from 1880. Related: Triply (adv.). Triple-decker is from 1940 of sandwiches and wedding cakes, 1942 of beds.